Neutral Temperature is a railroad term used to describe the amount of stress present in rail. The neutral temperature is the temperature at which the rail is neither in compression nor in tension. It is sometimes referred to as the stress-free temperature. Rail is made of steel and all steel expands as it heats up and contracts as it cools. As rail heats up past the neutral temperature, significant amounts of stress can build up in the rail. If enough stress builds up, the rail will buckle, this is commonly known as a sun-kink or rail buckle and is a major cause of train derailments worldwide. The opposite effect is when the rail contracts due to cooling too far below the neutral temperature causing rail breaks, also a major cause of derailments.
The problem that railroads face is there is no way to walk up to a piece of rail and know how much stress is present or what the neutral temperature currently is. There are sensors available that use standard strain gages and thermocouples to measure neutral temperature. Strain gages measure strain and from strain, stress can be computed using the modulus of elasticity, a material constant.
The major short-coming of strain gage based sensors is: Strain gages can only measure change in strain, in order to measure the absolute strain, they must be calibrated. In order to calibrate a strain gage, the current stress in the rail must be known. Once the current stress is known, an offset is calculated and applied to all future strain gage readings. The railroads have several methods by which they can determine the stress in the rail. 1) cut the rail, creating a stress-free condition or 2) using a device such as a Verse, a commercially available product that measures the stress in rail by picking it up with a cable and claw system and measuring how much force is required to move it. In order to use the Verse, the rail must be unclipped in 100 feet in each direction and the rail must also be in compression. Both of these methods are costly and time consuming for the railroads.